1. Technical Field
This invention relates to a wireless communication device for transmitting and receiving information by using a wireless frequency band of 10 [MHz] to 5 [GHz].
2. Description of the Prior Art
A baby monitor for hearing a baby's voice from a separated room (within the range from 5, 6 meters to 150 meters) as a device in a “toy commodity market” for children used in e.g., the interior of the United States of America is known as a conventional wireless communication device. In this baby monitor, a two-crystal system of two channels is adopted in a receiver and crystal control FM (frequency modulation) of two channels is adopted in a transmitter so that information transmission and reception corresponding to the two channels can be performed.
A monitor/hand-free mutual communication system is disclosed in U.S. Pat. No. 6,091,329 as a system having the two crystals on the receiving and transmitting sides of the wireless communication device. In the wireless communication device of this U.S. Pat. No. 6,091,329, an RF circuit from an antenna high frequency (RF) circuit to a demodulating circuit (FM ratio detector) is constructed by arranging six transistors and two demodulation wave detection diodes. In this circuit construction, crystal resonators are required in a local oscillating circuit by the number of channels as using frequencies. Namely, this wireless communication device has two crystal resonators on each of the transmitting and receiving sides to cope with the two channels, and selects each channel by a switch. The local oscillating circuit generally has a variable capacitor to finely adjust an oscillating frequency of about several [Hz] in each crystal resonator. With respect to such technical specification of the wireless communication device such as the baby monitor, for example, the wireless communication device is produced as a device conforming to the standard of America Federal Communications Commission (FCC) as an administrative committee in an information-communication field in the United States of America.
However, in the circuit construction of the conventional wireless communication device, the RF circuit from the antenna high frequency circuit to the demodulating circuit (FM ratio detector) is constructed by six transistors and two demodulation wave detection diodes, using no crystal resonator. Further, the band of an intermediate frequency amplifying circuit is widened to compensate an unstable characteristics in the local oscillator. Therefore, a problem existed in that receiving sensitivity is very bad. On the other hand, when a crystal resonator and a variable capacitor for finely adjusting the oscillating frequency of the oscillator is arranged, the range of the finely adjustable frequency is a narrow range of about several [Hz]. Therefore, a problem existed in that the oscillating frequency becomes unstable when the oscillating frequency exceeds this range.
Further, in the conventional wireless communication device, expensive crystal resonators (crystal oscillators), variable capacitors for finely adjusting the oscillating frequency, etc. are required by the number of channels for communicating information in the local oscillating circuit. When this channel number is two or more, a problem exists in that device cost is raised and device size is increased contrary to the need of compactness, etc.